RSRF recipe updates

[janusbrink]

The following changes should be agreed upon and updated in the DRLD for the RSRF recipe:

1. According to the METIS Calibration Plan, a set of WCU_OFF frames are a required input and stacked for use as a background image that is subtracted from the RSRF_RAW frames -> intermediate output IFU_RSRF_BACKGROUND.
2. The DRLD mentions obtaining RSRF_RAW frames at two different blackbody temperatures. The Calibration Plan suggests that this is most likely not required.
3. The Calibration Plan mentions using the pinhole mask and scanning the pinholes along and across the IFU slits. If this is indeed the plan, the DRLD should be updated to reflect this as well as how the resulting output should be combined (or not).